This invention relates to solid-state lasers and, more particularly, to a method for constructing such a laser so that it will operate in only its fundamental mode.
Extensive efforts have been directed in recent years at developing stripe-geometry junction lasers (for example, those of the double-heterostructure type). These solid-state devices operate on a continuous-wave basis at room temperature and, in addition, provide the capability of obtaining relatively high pulsed power outputs.
In a variety of applications of practical interest, it is desirable that such a laser be controlled to operate in only its fundamental or zero-order mode. In an optical communication system, the fundamental mode facilitates optical coupling between various components of the system (for example, efficient matching into singlemode fibers for dispersion-free propagation). Furthermore, where high optical power densities are desirable, such as in a laser machining or micrographics system, the fundamental mode is most desirable because it permits the simplest delivery of power from the laser to the image plane of the system.
Multimode operation of a double-heterostructure laser is typically characterized by the presence of plural competing laser spots on the output faces of the device. This phenomenon, often referred to as filamentation, is, as indicated above, generally undesirable.
Various techniques have been suggested for controlling filamentation in double-heterostructure lasers. Some of these require complex fabrication steps for forming a three-dimensional waveguide resonator and, moreover, may not permit the attainment of a laser cavity with a cross section sufficiently large to handle the needed peak power. An example of such a technique is described in "GaAs-Ga.sub.1.sub.-x A1.sub.x A.sub.s Buried Heterostructure Injection Lasers" by T. Tsukada, J. Appl. Phys., pages 4899-4906, November 1974.
Another suggested technique for controlling filamentation in a double-heterostructure laser involves the fabrication of a very small mirror on one of the output faces of the laser. This approach requires a practical instrumentality for defining the mirror and for accurately positioning it in line with the major filament or fundamental-mode-output spot of the laser.